1. Field of the Invention
The present invention relates to a capacitor for a semiconductor integrated circuit which uses a dielectric material and a method of manufacturing the same, and, more particularly, to a semiconductor integrated circuit device of a high density type which uses the aforesaid capacitor.
2. Description of the Related Art
A dynamic random access memory, hereinafter called a "DRAM", has been studied energetically in recent years because it has, as a storage device of a computer, advantages that it has a considerably large storage capacity and it is capable of operating at high speed. Under the aforesaid condition, there has been a desire of further raising the operation speed and the degree of integration. The technology about the DRAM has been in detail disclosed in, for example, "The Latest Very Large-Scale Integrated Circuit Handbook" edited by Yoichi Akasaka and three others and published by Science Forum.
FIG. 63 is a vertical cross sectional view which illustrates a memory cell portion of a typical DRAM. Each memory cell comprises a pair composed of a MOS transistor including a source S, a drain D, a source electrode 14, a drain electrode 301 and a gate electrode 11 formed on a Si substrate and a capacitor including a drain electrode 301, a dielectric material 302 and a plate electrode 303, so as to store 1-bit data depending upon the charge stored in the capacitor. A gate electrode 11 of the MOS transistor is connected to a word line which is connected to an X-decoder driver of a peripheral circuit. On the other hand, the source electrode 14 of the MOS transistor is connected to a bit line which is connected to a peripheral circuit such as a sense amplifier, a reading circuit, a writing circuit and the like.
A charge larger than 200 fC must be stored in the capacity of the capacitor in order to withstand an error (called a "soft error") taken place due to a charge generated by .alpha. rays. Assuming that the power supply voltage is 3 V, the capacitor requires a capacity of about 70 fF.
The capacity of the capacitor is expressed by the following Equation (1): ##EQU1## where C: capacity
.zeta..sub.0 : permittivity in vacuum PA1 .zeta..gamma.: relative permittivity PA1 S: area of electrode PA1 d: thickness of insulating film
As can be understood from Equation (1), the capacity of the capacitor is in proportion to the relative permittivity .zeta..gamma. of the insulating film and the electrode area S of the capacitor, but is in inverse proportion to the thickness d of the insulating film. Therefore, the surface area S of the electrode must be enlarged, the thickness d of the insulating film must be reduced and an insulating film having a large permittivity must be used in order to enlarge the capacity C of the capacitor. However, it is difficult to satisfactorily enlarge the electrode area S of the capacitor by the conventional mass production technology because the surface area of one memory cell of a highly-integrated DRAM is reduced. Therefore, a study for enlarging the surface area has been made, for example, as disclosed in 1991 Symposium on VLSI Technology Digest of Technical Papers P7-P13. As can be understood from this, a considerably complicated process has been required in order to enlarge the surface area. However, the thickness of the insulating film cannot be reduced satisfactorily because the puncture electric field must keep a required level.
On the other hand, a study for making the insulating film by a material having a large permittivity has been carried out as disclosed in, for example, P3 to P29 of Japanese Journal of Applied Physics, September 1991, Vol. 30, No. 9B, Ferroelectric Materials and their Applications. The materials having a large permittivity .zeta..gamma. are exemplified by Ta.sub.2 O.sub.5 and TiO.sub.2 each having a permittivity .zeta..gamma. of about 20 to 100, and ferroelectric materials having a perovskite type crystalline structure such as Pb (ZrTi)O.sub.3, (PbLa) (ZrTi)O.sub.3, BaTiO.sub.3, and SrTiO.sub.3 each of which has a permittivity larger than the aforesaid value.
The ferroelectric material has a so-called spontaneous polarization phenomenon in which it has a polarization although no electric field is applied thereto. The aforesaid material has a Curie temperature. The material has the spontaneous polarization in the case where the temperature is lower than the Curie temperature, while the material has no spontaneous polarization in the case where the temperature is higher than the Curie temperature. If the temperature is in the vicinity of the Curie temperature, the permittivity of the material becomes maximum, and the permittivity of some materials is larger than 10,000 at the aforesaid temperature. By solid-dissolving materials having different Curie temperatures, the Curie temperature can be shifted from the specific value of the material, the peak width of the permittivity with respect to the temperature can be widened, causing the dependency upon the temperature can be changed.
FIG. 64 is a graph which illustrates a typical spontaneous polarization. As shown in FIG. 64, the ferroelectric material takes place a phenomenon called the "spontaneous polarization" in which it has the polarization therein even if no electric field is supplied thereto. A technology about a ferroelectric memory which uses the aforesaid spontaneous polarization as a memory has been disclosed in Japanese Patent Publication(A) No. 63-201998, Japanese Patent Publication(A) No. 64-066897 and Japanese Patent Publication(A) No. 1-158691. Another technology about a highly-integrated DRAM in which a ferroelectric material (PZT) is used and capacitors are arranged three-dimensionally has been disclosed in U.S. Pat. No. 5,081,559.
As the applicable examples of the ferroelectric material other than the use as the capacitor, it has been variously used, for example, as an infrared ray sensor, and an electrooptical device, and the like. In the aforesaid electronic devices, the size and the thickness of the ferroelectric member have been reduced with the tendency of reducing the size and raising the degree of integration.
Since the perovskite type ferroelectric material usually has a very large relative permittivity and also has a large anisotropy, it is considered as a ferroelectric material. A thin film made of the ferroelectric material is, as disclosed in Japanese Patent Publication No. 1-80339, formed by an evaporation method, a sputtering method or a plasma oxidation method by utilizing the larger permittivity. It is necessary for the ferroelectric memory to form a capacitor of the ferroelectric material on the silicon substrate on which the transistors are formed. However, the ferroelectric material, the composition of which is expressed by ABO.sub.3, can easily be reacted with silicon and therefore it cannot be directly formed on the silicon or on the silicon oxide film. Therefore, a barrier layer must be formed in order to prevent the aforesaid reaction. Although noble metal such as Pt can be used as an excellent lower electrode because it has excellent barrier characteristics and small electric resistance, it cannot be subjected to a fine process such as etching.
In order to reduce the size of a computer and to raise the operation speed of the same, the storage device included by the computer must be highly integrated. Therefore, there is a desire of reducing the storage cell per bit in order to reduce the size of the semiconductor device for use as the internal storage device. Accordingly, the capacitor for use as the dynamic type memory or the ferroelectric nonvolatile memory must be reduced, causing a necessity to arise in that the permittivity of the ferroelectric material for use in the capacitor must be raised and another necessity to arise in that the spontaneous polarization value of the ferroelectric material shown in FIG. 64 must be enlarged.
FIG. 65 illustrates the structure of the conventional capacitor formed in such a manner that noble metal is used as the base electrode. In the case where noble metal such as platinum, palladium is used as the material which does not form the oxide having a low permittivity, the metal oxide is not formed if the film is sufficiently thick, resulting in a ferromagnetic film revealing excellent crystallinity and a high permittivity to be formed. However, even if the insulator having a high effective permittivity can be formed by the aforesaid technique, noble metal such as platinum must be subjected to a process such as ion milling or wet etching because it cannot be subjected to a reactive ion etching process or a dry etching process. Therefore, a problem arises in that high integration cannot be realized.
Curve A shown in FIG. 4 is a graph which illustrates the relationship between the thickness of the ferroelectric film and the capacity. FIG. 4 illustrates results of an experiment carried out about the relationship between the capacity of the capacitor and the thickness of the BaTiO.sub.3 film in the case where BaTiO.sub.3 is employed as the ferroelectric material, and the electrode area is made 1.times.1 .mu.m.sup.2. In the case where the thickness of the film is thinned as described above, a problem arises in that a large leakage current flows between the electrodes and therefore the charge holding characteristics deteriorate. In the case where the capacitor of the aforesaid type is used as the capacitor of the DRAM, another problem arises in that the thickness cannot be reduced because a sufficient charge cannot be left at the time of the reading operation due to a fact that the charge stored at the time of the writing operation decreases. Hence, the area of the device cannot be reduced because the area required for the capacity increases in the case where a desired capacity is intended to be obtained.
As described above, a material having a satisfactorily large permittivity has not been obtained for the purpose of realizing an application of the ferroelectric material as a device although there has been a desire of obtaining an excellent oxide ferroelectric thin film.